CMS user AJAX checking is case sensitive

The AJAX checking for CMS username/email is case sensitive but it shouldn't.
Eg. there is a username "testuser" existing. If you create an account in Joomla front end CiviMember registration with username "testUser", the username checking button will tell you "The username is currently available".

It should be case insensitive since Joomla/Drupal only allow lower case username.